SAN JUAN, Puerto Rico — Navy officers detained 56 demonstrators Saturday after they invaded the U.S. Navy bombing range on Vieques island and demanded that the Navy leave.

Among those detained on the small Puerto Rican island were 20 doctors — from surgeons to general practitioners — who said in a statement: "We are a group representing Puerto Rican doctors who are concerned about the health and future of the people of Vieques."

All the protesters were cited to appear in federal court to be charged with trespassing, Navy spokesman Robert Nelson said. They were subsequently released. Nelson said he did not know when they will appear in court.

Decades of resentment over the Navy's presence on Vieques boiled over in April 1999, when a jet dropped two 500-pound bombs off-target and killed a civilian security guard. It was the first fatality in 60 years since the Navy took over two-thirds of the 20-mile-long island, dislodging thousands of residents and using part of the island as a practice bombing range. Some 9,300 civilians now live on Vieques.

People's anger was compounded when the Navy admitted it had illegally and accidentally fired 263 armor-piercing bullets tipped with depleted uranium on the range in February 1999. Puerto Rican officials said they were never informed.

Dozens of protesters occupied the range for a year to prevent more bombing. They were forced out in a federal raid May 4, and the Navy resumed training under a deal that calls for a referendum before February 2002 and the use of non-exploding practice bombs until after the vote.

If the Navy loses the referendum, it would have to leave the island by May 2003, giving up a site it says is crucial for the kind of realistic training that saves lives in combat. But many Vieques residents accuse the Navy of contaminating their environment, destroying fishing grounds and coral reefs, keeping away tourists and contributing to a 14 percent unemployment rate. Dozens are filing lawsuits claiming their health problems, from respiratory illnesses to cancers, are related to the military exercises.